1. Field of the Invention
This invention relates to a charging member which is disposed in contact with a charging object member and charges the charging object member electrostatically upon application of a voltage. This invention also relates to a process cartridge and an electrophotographic apparatus which have such a charging member.
2. Related Background Art
In recent years, a contact charging type charging means is being employed as a charging means used in image-forming apparatus such as an electrophotographic apparatus and an electrostatic recording apparatus. Contact charging is a method in which a charging object member is charged electrostatically to a stated polarity and potential by applying a voltage to a charging member disposed in contact with the charging object member, and has advantages such that the voltage of power sources can be set low, corona products, such as ozone, can be made to occur less frequently, and the system can be set up in simple construction to enable a cost reduction.
The voltage is applied to the charging member by a method in which only direct current is applied (a DC application method) or by another method in which an oscillating electric field (an electric field where the voltage value changes periodically with time) having a peak-to-peak voltage which is at least twice the voltage at which the charging of the charging object member is started is formed across the contact charging member and the charging object member to charge the surface of the charging object member (an AC application method). The latter can perform more uniform charging.
By the shape and form of the charging member to be brought into contact with the charging object member, units for the contact charging are roughly grouped into a roller type charging assembly making use of a roller member (charging roller) as its charging member (e.g., Japanese Patent Application Laid-Open No. 56-91253), a blade type charging assembly making use of a blade member (charging blade), and a brush type charging assembly making use of a brush member (charging brush) (e.g., Japanese Patent Application Laid-Open No. 64-24264).
The charging roller is rotatably supported with bearings and kept into pressure contact with the charging object member at a stated pressure, and is follow-up rotated with the movement of the charging object member. Also, the charging roller is usually a multi-layered structural member comprising a mandrel provided at its center as a substrate, a conductive elastic layer provided in the form of a roller on the periphery of the mandrel, and an intermediate layer and a surface layer which are further provided on the periphery of the elastic layer.
Of the above layers, the mandrel (metal layer) is a rigid body for maintaining the shape of the roller and at the same time has the function as a power supply electrode.
The elastic layer is required to have a volume resistivity of 1xc3x97102 to 1xc3x971010 xcexa9xc2x7cm and to be elastically deformable so as to ensure uniform contact with the charging object member. Accordingly, vulcanized (cured) rubbers are usually used therefor which are endowed with conductivity and have a flexibility of 70 degrees or below in rubber hardness (JIS A). Also, in conventional charging rollers, there have been a foam type and a solid type, the former making use of a rubber foam (or sponge rubber) as the elastic layer and the latter not making use of a rubber foam. The above AC application method has had a problem that a force acts between the charging roller and the charging object member because of the action of oscillating electric field and makes the charging object member vibrate to cause noise. Hence, it is considered preferable to use as the elastic layer a rubber foam as having a lower hardness.
The intermediate layer has the function to relax any compression of the elastic layer and the function to prevent the bleeding of any softening agents, such as oils and plasticizers, used in order to lower the hardness of the elastic layer; the latter function enhancing the freedom of materials used in the surface layer. The intermediate layer may usually have a surface resistivity of from 1xc3x97105 to 1xc3x97102 xcexa9/square, and has conventionally been formed by coating the elastic layer with a conductive coating material or covering it with a seamless tube.
The surface layer has the function to improve the uniformity in charging the charging object member and prevent any leak from being caused by pinholes of the charging object member surface, and also has the function to prevent toner particles, paper dust and so forth from sticking to the surface. The surface layer may usually have a surface resistivity of from 1xc3x97105 to 1xc3x971013 xcexa9/square, and, like the intermediate layer, has been formed by coating with a conductive coating material or covering with a seamless tube.
As a method of forming the elastic layer, a method is known in which an uncured (unvulcanized) and unfoamed semiconductive foamable rubber material is extruded in the form of a tube by means of an extruder, followed by heating in a curing furnace or the like to cure and foam the extruded product to form a semiconductive foamed rubber tube, and further a mandrel coated with a hot-melt adhesive is inserted to this semiconductive foamed rubber tube, followed by heating to bond the mandrel and the semiconductive foamed rubber tube together. This method, however, has had a problem that it requires so large a number of steps as to result in a high production cost.
As a countermeasure therefor, Japanese Patent Application Laid-Open No. 10-221930 discloses a method in which a mandrel coated with an adhesive is passed through a crosshead die of an extruder to dispose the uncured and unfoamed semiconductive foamable rubber material on the periphery of the mandrel, and thereafter the curing (vulcanization) and foaming of the rubber material and the bonding of the mandrel and the semiconductive foamed rubber tube together are simultaneously carried out using a vulcanizer or a continuous curing furnace so that the number of process steps can be cut down.
However, where in this method a semiconductive foamable rubber composition having a high foaming expansivity is used, the resultant semiconductive foamed rubber tends to lift partly from the mandrel as a result of the foaming of the semiconductive foamable rubber, and hence the composition can not be foamed at a high expansivity. Thus, this method has had problems such that any foamed rubber having a sufficiently low-hardness can not be obtained and the part where the rubber has lifted from the mandrel causes uneven charging.
As another method of producing a foamed robber having a low hardness, a method is also known in which a softening agent is added to the rubber composition in a large quantity. However, this method has had problems such that, when the softening agent is merely added in a large quantity, a slip may occur during kneading to make it impossible to carry on the kneading, and, when the softening agent is added little by little in a large quantity so as not to cause the slip, it takes long time to carry out the kneading, resulting in a cost increase.
As for a method of covering with the tube, a method is available in which an intermediate layer tube is inserted to a cylindrical mold and fastened to both ends of its inner wall, and the space between the tube and the mold inner wall is evacuated to bring the tube into close contact with the mold inner wall, in the state in which an elastic roller, obtained by forming an elastic layer on the periphery of a mandrel, is inserted under the application of a pressure. Covering with a surface layer tube may also be carried out in the same way.
However, where such a charging roller, having cover layers formed by superposing single-layer tubes prepared in plurality, by fitting them externally one by one to the elastic layer, is used for electrophotography, the charging roller is kept in pressure contact with the charging object member at a stated pressure under application of a load to both ends of the charging roller and is follow-up rotated with the movement of the charging object member. In such a case, a torsional force due to a difference in pressure contact force between the ends and the middle may act to cause the superposed tubes to slip off at their interface, so that the tubes become twisted. As the result, a difference in image density may appear at the part corresponding to the ends and the middle. This phenomenon tends to occur more as the elastic layer is softer and the pressure contact force at the both ends is greater.
As a method of preventing the tubes from being thus twisted, the inner diameter of each tube may be made larger with respect to the outer diameter of the elastic layer so that the tubes may tighten the elastic layer at a greater force. However, as an ill effect, the elastic layer may have so high an apparent hardness as to tend to make a large charging noise.
In addition, where the tubes are made of a thermoplastic resin or the elastic layer has a high hardness, any elongation of the tubes makes the charging roller have a large external diameter, so that its resistance value may become too high to charge the charging object member sufficiently. This phenomenon may more remarkably occur as the elastic layer is harder.
An object of the present invention is to provide a charging member which has an elastic layer having a sufficient elasticity and a sufficient adherence to the mandrel and also can exhibit very uniform charging performance without causing any twist of the cover layer on the elastic layer.
Another object of the present invention is to provide a process cartridge and an electrophotographic apparatus which have such a charging member.
To achieve the above objects, the present invention provides a charging member comprising a conductive mandrel, a semiconductive foamed elastic layer provided on the periphery of the mandrel, and a functional double-layer film provided on the periphery of the semiconductive foamed elastic layer, wherein;
the semiconductive foamed elastic layer is a layer formed by making the mandrel and a semiconductive rubber composition standing uncured and unfoamed pass through a crosshead die of an extruder to set the composition on the periphery of the mandrel, followed by curing and foaming;
the semiconductive rubber composition having a Mooney viscosity of from 15 to 30 and having a curing percentage of 40% or less when the foaming pressure reaches 50%; and
the functional double-layer film being a double-layer tube having a thin layer such that a tube formed out of only the layer is hard to use for covering.
The present invention also provides a process cartridge and an electrophotographic apparatus which have the above charging member.